In factories, power plants, and the like, gasses are used in large quantities as chemical source materials and fuels. Gasses used as chemical source materials and fuels are often flammable or toxic, and their leaking outside may cause air pollution and fire and explosion accidents. Even with gasses with low toxicity, their presence at a high concentration may cause a low oxygen concentration, leading to asphyxiation and other accidents.
In facilities seen as factories and power plants where gasses are used, the piping and equipment through which gasses pass are checked carefully on a regular basis, and in addition gas leak detectors incorporating gas-sensitive elements are installed to watch for gas leaks. However, depending on the properties (such as specific gravity) of a gas and the current of air in a space, even if a plurality of gas leak sensors are installed, the gas may elude them and continue leaking, often resulting is inaccurate gas leak detection.
As a solution, there are proposed optical gas detection devices that utilize the optical absorption property peculiar to gasses. In an optical detection device, a space in which to detect a gas leak is shot with an image sensor; an increase or decrease under the influence of the gas in the black body radiation produced by an object constituting the background is detected, and thereby whether the gas is present or absent is determined. More specifically, image data in which the wavelengths influenced by the gas (for example, wavelengths absorbed or emitted by the gas) are cut with a filter or the like is shot with the image sensor. Then, with the filter replaced or removed, image data including the wavelengths influenced by the gas is shot with the image sensor. Then, the two sets of image data are compared with each other to detect a gas leak. Herein, radiated electromagnetic waves based on the temperature on the object surface is referred to as black body radiation.
Image sensors used in such optical gas detection devices divide into cooled sensors and uncooled sensors. Cooled sensors have the advantages of fast response and high sensitivity, but require a special cooling device. Thus, using a cooled sensor makes an optical gas detection device complicated and cumbersome.
By contrast, uncooled sensors require no cooling device, and thus using one helps make an optical gas detection device simple and compact. On the other hand, in an uncooled sensor, each pixel has a fine three-dimensional structure. This fine three-dimensional structure is prone to variations ascribable to the manufacturing process, and thus pixel characteristics are prone to variations. In an optical gas detection device employing art uncooled sensor, correction is performed to reduce variations in characteristics among the pixels of the uncooled sensor (see, for example, Ex-PCT Japanese Patent Application Publication No. 2010-522317).
In the remote optical gas detection device disclosed in Ex-PCT Japanese Patent Application Publication No. 2010-522317, a black body is arranged, the path of incoming light is obstructed with the black body (an infrared lens is obstructed), and an image sensor is corrected for heat drift; in this way, a shot image can be made uniform again. Thus, even when an uncooled sensor, which has lower accuracy than a cooled sensor, is used as an image sensor in an optical gas detection device, it is possible to detect a gas with high accuracy.